1. Field of the Invention
The present invention relates generally to a data transmission/reception apparatus and method in a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to a data transmission/reception apparatus and method suitable for high-speed data transmission requiring an adaptive modulation and coding scheme.
2. Description of the Related Art
A mobile communication system has evolved from an early voice communication system that chiefly provides a voice service into a high-speed, high-quality radio data packet communication system that provides a data service and a multimedia service. Standardizations on HSDPA (High Speed Downlink Packet Access) and 1×EV-DV (Evolution Data and Voice) are separately made by 3GPP (3rd Generation Partnership Project) and 3GPP2 (3rd Generation Partnership Project 2) in an attempt to find out a solution for a high-speed, high-quality radio data packet transmission service of 2 Mbps or over in a 3rd generation mobile communication system. Meanwhile, a 4th generation mobile communication system is proposed to provide a high-speed, high-quality multimedia service superior to that of the 3rd generation mobile communication system.
In radio communications, a principal factor of impeding the high-speed, high-quality data service lies in a channel environment. The radio channel environment is frequently changed due to a variation in signal power caused by white nose and fading, shadowing, Doppler effect caused by a movement of and a frequent change in speed of a UE (User Equipment), and interference caused by other users and a multipath signal. Therefore, in order to provide the high-speed radio data packet service, there is a need for an improved technology capable of increasing adaptability to the variation in the channel environment in addition to the general technology provided for the existing 2nd or 3rd generation mobile communication system. A high-speed power control method used in the existing system also increases adaptability to the variation in the channel environment. However, both the 3GPP and the 3GPP2, carrying out standardization on the high-speed data packet transmission system, make reference to AMCS (Adaptive Modulation/Coding Scheme) and HARQ (Hybrid Automatic Repeat Request).
The AMCS is a technique for adaptively changing a modulation scheme (or technique) and a coding rate of a channel encoder according to a variation in the downlink channel environment. Commonly, a UE acquires channel quality information of the downlink by measuring a signal-to-noise ratio (SNR), and transmits the channel quality information of the downlink to a Node B over an uplink. The Node B predicts a channel condition of the downlink channel based on the channel quality information of the downlink, and designates a proper modulation scheme and coding rate based on the predicted value. The modulation schemes considered in the HSDPA and 1x-EVDV include QPSK (Quadrature Phase Shift Keying), 8PSK (8-ary Phase Shift Keying), 16QAM (16-ary Quadrature Amplitude Modulation) and 64QAM (64-ary Quadrature Amplitude Modulation), and the coding rates considered in the HSDPA and 1×-EVDV include ½ and ¾. Therefore, an AMCS system applies the high-order modulation schemes (16QAM and 64QAM) and the high coding rate ¾ to the UE having a good channel condition, and applies the low-order modulation schemes (QPSK and 8PSK) and the low coding rate ½ to the UE having a poor channel condition. Commonly, a UE with a good channel condition is a UE located in the vicinity of a Node B, and a UE with a poor channel condition is a UE located in a boundary of a cell. Compared with the existing high-speed power control method, the AMCS decreases an interference signal, improving average system performance.
The HARQ is a link control technique for correcting an error by retransmitting the errored data upon occurrence of a packet error at initial transmission. Generally, the HARQ is classified into Chase Combining (CC), Full Incremental Redundancy (FIR), and Partial Incremental Redundancy (PIR). The CC is a technique for transmitting a packet such that the whole packet transmitted at retransmission is equal to the packet transmitted at initial transmission. In this technique, a receiver combines the retransmitted packet with the initially transmitted packet. By doing so, it is possible to increase reliability of coded bits input to a decoder, thus resulting in an increase in the entire system performance. Combining the two same packets is similar to repeated coding in terms of effects, so it is possible to increase a performance gain by about 3 dB on the average. The FIR is a technique for transmitting a packet comprised of only the parity bits generated from the channel encoder instead of the same packet, thus to improve a coding gain of a decoder in the receiver. That is, the decoder uses the new parity bits as well as the initially transmitted information during decoding, resulting in an increase in the coding gain. The increase in the coding gain improves performance of the decoder. It is well known in a coding theory that a performance gain by a low coding rate is higher than a performance gain by repeated coding. Therefore, the FIR is superior to the CC in terms of only the performance gain. Unlike the FIR, the PIR is a technique for transmitting a combined data packet of systematic bits and new parity bits at retransmission. The PIR obtains the similar effect to the CC by combining the retransmitted systematic bits with the initially transmitted systematic bits during decoding. Further, the PIR obtains the similar effect even to the FIR by performing decoding using the parity bits. The PIR has a coding rate slightly higher than that of the FIR, showing medium performance between the FIR and the CC. However, the HARQ should be considered in the light of not only the performance but also the system complexity such as a buffer size and signaling of the receiver, so it is not easy to determine which HARQ technique best applies.
The AMCS and the HARQ are separate techniques for increasing adaptability to the variation in the link environment. However, it is possible to greatly improve the system performance by combining the two techniques. That is, if a modulation scheme and a coding rate proper for a downlink channel condition by the AMCS, then data packets corresponding thereto are transmitted.
FIG. 1 illustrates a structure of a conventional transmitter for high-speed packet data transmission. Referring to FIG. 1, a channel encoder 10 can realize AMCS and HARQ under the control of a controller 18. The channel encoder 10 is comprised of an encoder and a puncturer. If data proper to a data rate is applied to an input terminal of the channel encoder 10, the encoder performs encoding and provides the coded bits to a channel interleaver 14, in order to reduce a transmission error rate. The channel interleaver 14, a device for coping with a fading channel, separates bits constituting particular information (e.g., one word of a voice signal) from one another as far as possible, thereby decreasing a probability that the information will be lost at the same time. The interleaved signal is modulated into a symbol by a modulator 16 before being transmitted. A receiver then performs error decision on a received packet and informs the transmitter of the error decision result. If there is no error, the transmitter transmits a new packet. Otherwise, if there is an error, the transmitter retransmits the previously transmitted data. For the retransmission, the transmitter may transmit the same transmission data as initially transmitted data according to the CC of the HARQ, or transmit new channel-coded data according to the FIR or PIR of the HARQ. In the next generation mobile communication system, a more powerful coding technique is required for reliable transmission of high-speed multimedia data. A turbo encoder is a typical example of the channel encoder 10. It is known that a channel coding technique using the turbo encoder shows performance most approximative to the Shannon limit in light of a bit error rate (BER) even at a low SNR. This channel coding technique is adopted for the HSDPA and the 1xEV-DV by the 3GPP and the 3GPP2.
An output of the turbo encoder can be divided into systematic bits and parity bits. The systematic bits mean actual data to be transmitted, and the parity bits mean a parity signal added to correct an error generated during transmission at the receiver. Though not illustrated in FIG. 1, the channel encoder 10 includes a puncturer in a CDMA mobile communication system. The puncturer selectively punctures the systematic bits or parity bits among the output of the channel encoder 10, thereby satisfying the determined coding rate and demodulation order.
An operation of the channel encoder 10 will be described in detail. An input signal applied to the channel encoder 10 is output as a stream X of systematic bits. A first internal encoder of the channel encoder 10 encodes the input signal, and outputs two different streams Y1 and Y2 of parity bits. The input signal is also provided to an internal interleaver of the channel encoder 10. A signal interleaved by the internal interleaver is output as a stream X′ of interleaved systematic bits, and at the same time, provided to a second internal encoder of the channel encoder 10. The second internal encoder encodes the interleaved signal and outputs two different streams Z1 and Z2 of parity bits. The streams X and X′ of systematic bits, and the streams Y1, Y2, Z1 and Z2 of parity bits are provided to a puncturer in the channel encoder 10. The puncturer punctures the streams X and X′ of interleaved systematic bits, and the different streams Y1, Y2, Z1 and Z2 of parity bits using a puncturing pattern selected by a control signal from the controller 18, thereby outputting only desired systematic bits and parity bits. The puncturing pattern used in the puncturer is provided from a puncturing pattern generator. The puncturing pattern depends upon a coding rate and the HARQ type. That is, if the HARQ type is CC, the puncturer punctures the coded bits such that the systematic bits and the parity bits have a fixed combination according to a prescribed coding rate, so the transmitter can transmit the same packet at each transmission. However, if the HARQ type is IR (Incremental Redundancy), the puncturer punctures the coded bits using a combination of the systematic bits and the parity bits at initial transmission, and determines whether to include the systematic bits at retransmission according to whether the IR is PIR or FIR. However, the puncturer may puncture the coded bits using various combinations of the systematic bits no matter whether the IR is PIR or FIR, thereby increasing the entire coding gain.
The systematic bits and the parity bits output from the channel encoder 10 are applied to the interleaver 14. The interleaver 14 interleaves coded bits comprised of the systemic bits and the parity bits. Therefore, the systematic bits and the parity bits are combined into one bit stream. The stream of the interleaved coded bits is applied to the modulator 16. The modulator 16, under the control of the controller 18, modulates the stream of coded bits by a prescribed modulation scheme and outputs modulation symbols. The modulation symbols output from the modulator 16 are distributed by a transmission antenna assigner 20 to a plurality of antennas constituting an antenna array. The distributed modulation symbols are transmitted through the associated antennas.
FIG. 2 illustrates a structure of a receiver corresponding to the transmitter described in conjunction with FIG. 1. Referring to FIG. 2, modulation symbols are received through a plurality of reception antennas constituting one antenna array, and the modulation symbols received through the associated antennas are provided to a channel estimation and antenna data classification block 48. The channel estimation and antenna data classification block 48 multiplexes the modulation symbols received through the reception antennas into one stream of modulation symbols. The stream of the modulation symbols is provided to a demodulator 50, and the demodulator 50 demodulates the stream of modulation symbols into a stream of coded bits by a modulation scheme corresponding to the modulation scheme used in the transmitter. The stream of coded bits are provided to a deinterleaver 54, and the deinterleaver 54 deinterleaves the stream of coded bits according to the interleaving pattern used in the transmitter. The stream of the deinterleaved coded bits is provided to a channel decoder 56, and the channel decoder 56 decodes the stream of the deinterleaved coded bits under the control of a controller 58 and outputs the decoded data stream as received data.
Commonly, in the case where errors occur in transmission data at a prescribed rate in a transmitter and a receiver for high-speed packet data transmission, errors generated in systematic bits exert more influence on entire performance of the mobile communication system, compared with errors generated in parity bits. Therefore, assuming that the same error rate is maintained as a whole, if the errors generated in the parity bits are larger in number than the error generated in the systematic bits, the receiver can perform decoding more accurately. That is, the systematic bits have more influence on the decoder compared with the parity bits. The reason is because the parity bits are redundant coded bits added to correct transmission errors during decoding.
The interleaver 14 in the transmitter of the conventional mobile communication system performs symbol interleaving regardless of priority (or importance) of the systematic bits and the parity bits. In other words, the conventional transmitter mixes the systematic bits and the parity bits, segments the mixed data bits according to transmission antennas of an antenna array, and transmits the segmented data bits through the associated transmission antennas. In this case, the transmission antennas have different transmission capabilities. Therefore, if a particular transmission antenna has a poor transmission capability, the systematic bits and the parity bits have a similar error rate, affecting the entire system performance. This means that the system performance becomes worse than when errors occur only in the parity bits. Therefore, there is a demand for a technique for decreasing a probability that errors will occur in systematic bits by taking into consideration a channel condition for the signals transmitted through the transmission antennas, thereby increasing the entire system performance.
Further, in a mobile communication system performing data transmission and reception using multiple antennas, in the case where transmission antennas have a similar channel condition, even though the transmission data is separated into systematic bits and parity bits before being transmitted, a performance gain may not occur. In this case, it is possible to improve system performance by assigning (or mapping) the systematic bits to the bits corresponding to positions more resistive to an error among the bits constituting a symbol and assigning the parity bits to the bits corresponding to positions relatively susceptible to an error, during modulation.
However, the above-stated techniques for improving performance of the mobile communication system have been used separately only. That is, in a mobile communication system using multiple antennas, there is not a case where a channel condition for each transmission antenna is applied using both techniques.
The conventional HARQ and AMCS techniques have contributed to an increase in entire system performance in high-speed packet communications. In addition, many attempts are still being made for an improved method. For example, there has been proposed a method for changing a level of the AMCS when a condition of a reception channel is changed during retransmission. That is, it is necessary to select an optimal transmission method according to a channel condition at initial transmission and retransmission.
In addition, there has been proposed a method for increasing a data rate by increasing the number of transmission/reception antennas used in Node Bs and UEs. In this case, since the transmission antennas have different transmission characteristics, future studies should be made into a transmission method considering the different transmission characteristics.